The invention disclosed herein relates to a method and apparatus for filling beverage cans in which the cans are pre-pressurized with an inert gas before being filled with a beverage drawn from a tank which is pressurized with an inert gas.
It is known that to prevent premature spoilage and a change in the taste characteristics of a beverage in a can, the amount of air remaining in a can after it is filled with a beverage must be minimized. When filling a beverage can, therefore, it is common practice to evacuate the can and then pre-pressurize it with an inert gas before filling it with the beverage. Evacuating, pre-pressurizing and filling a can is not a straight forward procedure, however, because special precautions must be taken to avoid having the thin wall of the can deformed by the pressure differential between the inside of the can and the atmosphere.
A can filling method which has been in use in recent years provides that an inert gas such as CO.sub.2 be admitted to the can through a differential pressure chamber whereupon the can is pre-pressurized to a pressure below that of the pressure of the gas which exists above the beverage in the storage tank. The final pre-pressurization takes place through a connection established to the inner atmosphere of the tank by means of the tube in the center of the filler valve which is otherwise known as the gas return line. The disadvantage of this method is that during pre-pressurization of the can with CO.sub.2 gas, the air previously located in the can remains there. In other words, the air in the can is at first diluted with CO.sub.2 gas. It is therefore not possible with this method to achieve a low air concentration in the can. The proportion of air in the can is even higher than that in the storage tank. Since the inert gas and air mixture is passed from the inside of the can into the tank during the can filling procedure, the inert gas in the tank becomes more and more diluted with air.
In another can filling machine which is in current use, the inside of the can is flushed or purged prior to being filled with the CO.sub.2 and air mixture derived from the atmosphere of the storage tank. Next, since the can is sealed to the filler valve, it is pre-pressurized with the gas and CO.sub.2 mixture derived from the storage tank through the above mentioned gas return line. Even with very high CO.sub.2 concentration on the inside of the storage tank, it is barely possible to achieve with this method a CO.sub.2 concentration of more than 80% in the can.